New evidence suggests that the evolution of life on Earth not in
shallow pools on the surface, but in the torrid depths of the planetary
crust. And the planet in question might not even be the Earth...

by Paul Davies

For centuries, people thought life was created in the Garden of
Eden by God. Then along came Charles Darwin with his theory of evolution,
and spoilt that cosy image. Darwin gave a convincing account of
how all life on Earth has gradually evolved from simple microbes.
However, he left open the question of how life got started in the
first place. Now scientists are sure they are close to solving that
mystery too, and the answer looks set to inflame passions once again.

Chemists have long tried to make life in a test tube, by simulating
the conditions of the primeval Earth. Envisaging an ancient pond
laced with minerals and bathed in solar radiation, researchers pinned
their hopes on finding a chain of reactions that would transform
a lifeless chemical soup into a primitive organism.

Chicago chemist Stanley Miller blazed the trail in the early 1950s
by zapping a noxious brew of gas and water with electricity. Others
have tried different formulae. Unfortunately, after fifty years of
experimentation, the results are disappointing. Some of the simpler
building blocks of life, like amino acids, are readily made, but the
building itself - a living cell - remains as elusive as ever.

Deep-sea Life

Now a radical new theory could explain the lack of progress. Scientists
increasingly suspect that the tepid pool scenario is wrong, and
that life didn't start on the Earth's surface at all. This change
of mind stems from the startling discovery of bizarre micro-organisms,
dubbed superbugs or extremophiles, that inhabit some of our planet's
most extreme environments.

American chemist Stanley Miller surveys the contents of a flask
where primitive gases have been zapped by an electric spark.
For many years, scientists assumed this was how life started
on Earth: now, many have doubts.

photo
- Hencoup Enterprises

In the late 1970s, a research submarine called Alvin was sent to
the bottom of the Pacific Ocean to explore a string of volcanic
vents known as 'black smokers'. These chimneys on the sea floor
spew forth superheated water rich in dusky chemicals. The investigators
were astonished to find many exotic life forms clustered around
the searing effluent in pitch darkness, including weird-looking
crabs and giant tube worms.

Most remarkable of all were bacteria that inhabited the hot zone
itself, thriving in the disgorging fluids at temperatures in excess
of 100oC. Previously, nobody believed that any life could
survive above the normal boiling point of water. These superbugs
lie at the base of the food chain. They are primary producers, turning
inorganic material from the hot vents directly into biomass, without
the need for sunlight.

Subterranean Bugs

Alvin's discovery turned out to be just the tip of the iceberg.
In the 1980s, Thomas Gold, an astrophysicist from Cornell University,
New York, supervised an oil drilling project in Sweden. On examining
the rock cores from several miles down, Gold was amazed to find
unmistakable signs of life.

At the time, the idea that something could live so deep in the
Earth's crust was laughed at. There have been many stories about
life in the underworld, from the Greek fable of Orpheus to Jules
Verne's Journey to the Centre of the Earth. Few scientists
took the possibility seriously. But Gold's claim was confirmed when
other drilling projects yielded similar results. Biologists in the
United States began extracting live microbes from miles beneath
North Carolina and the Columbia River basin. Environmentalists became
concerned that subterranean superbugs might eat through containment
vessels of buried nuclear waste, causing leaks.

Meanwhile, the international Ocean Drilling Programme retrieved
rocks from far below the seabed, and the story was the same. The
basalt of the ocean floor is teeming with microbes too. Some of
them have been cultured in the laboratory by John Parkes of the
University of Bristol, who believes that some species can withstand
temperatures as high as 169oC. It is beginning to look
as if there may be as much biomass inside the Earth as there is
on the surface.

Our Ancestors?

The full significance of the deep-living microbes became clear
only after their innards were analysed, following the pioneering
work of Karl Stetter at the University of Regensburg in Germany.
By sequencing the superbug's genes, microbiologists can construct
a sort of family tree linking them with normal bacteria. The results
came as a complete surprise. It turns out that the oldest and deepest
branches of the tree of life are all occupied by heat-loving superbugs.
In other words, the microbes residing deep within the Earth are
among the world's oldest surviving organisms. In effect, they are
living fossils, having changed little since the dawn of time.

Mono Lake in California is thick with salt and - in places -
almost boiling. Yet it thrives with microbes, closely related
to the earliest life-forms on Earth.

photo
- Jerre Goldin

To some researchers these discoveries spell out a fascinating message.
It suggests that life was incubated in the volcanic depths of the
Earth, in pressure-cooker conditions, and migrated to the cooler surface
zone only much later. This theory neatly meshes with what we know
about the Earth's history. The Solar System is four and a half billion
years old. For almost a billion years after the planets formed they
were pounded mercilessly by giant asteroids and comets. A record of
this primordial violence is etched on the face of our nearest neighbour
in space - the Moon - which is pockmarked with countless large craters.

The biggest impacts would have blasted away the Earth's atmosphere
and swathed the globe in incandescent rock vapour. The heat pulses
were fierce enough to boil the oceans dry and sterilise the exposed
land to a depth of half a mile or more. Not even superbugs could
survive such cataclysmic episodes unprotected. Yet, paradoxically,
there are traces of relatively advanced life in ancient rocks from
Greenland dated at over 3.85 billion years - a time before this
massive cosmic bombardment had abated. But if the 'comfort zone'
of heat-loving microbes was deep enough, they could shelter from
the cosmic barrage in the torrid strata of the Earth's crust, beyond
the reach of even the fiercest heat pulses.

Superbugs on Mars

Mars today is a frozen desert « yet dried-up channels and
crater erosion hints that the Red Planet was once warmer and
drier, with rivers and possibly oceans. Was this, too, a cradle
of life?

photo
- NASA

If the theory is right, and life did begin deep within the Earth,
it may also have got going beneath the surfaces of other planets too.
Mars is an obvious candidate. When two Viking space probes landed
there in 1977 they scooped up some dirt to test for biological activity.
No clear evidence was found, and most scientists pronounced the Red
Planet dead. With hindsight, this outcome wasn't surprising, since
the surface of Mars is a freeze-dried desert bathed in ultra-violet
radiation that would prove lethal to almost all known organisms. However,
beneath the hostile surface, conditions may be more congenial for
life. Geothermal heat will have melted the permafrost to create reservoirs
of liquid brine similar to those beneath the Earth's sea bed. So there
might be superbugs lurking below the harsh Martian terrain.

Although today Mars is cold and dry, in the remote past it was
warm and wet, and not unlike Earth. It had rivers and glaciers and
possibly a large ocean. Life may well have spread to the planet's
surface and briefly flourished in the Martian spring, before the
atmosphere leaked away and the temperature plunged. It's possible
these ancient organisms left fossils in the surface rocks, where
they may be discovered by forthcoming missions to the Red Planet.

The infamous
"microbes from Mars". Magnified 100,000 times the
interior of a meteorite from Mars (coloured red) reveals intriguing
bug-like shapes (blue). Researchers are stilling arguing whether
these are indeed fossilised bacteria.

photo
- NASA

A few years ago, some scientists claimed to find fossilised microbes
within a Martian meteorite collected in Antarctica. Although the jury
is still out on that, it no longer seems so fanciful to speculate
that Mars has, or at least once had, some form of life.

If there is, or was, life on Mars, then it raises a fascinating
possibility. Back in the early history of the Solar System, the
heavy bombardment that made conditions on the planets so dangerous
for surface life would also have kicked vast numbers of rocks out
into space. Many Mars rocks must have reached Earth during our planet's
history, and many ejected Earth rocks will have hit Mars. Could
hardy superbugs dwelling within these rocks have hitched a ride
through space and taken up residence on arrival?

Current evidence strongly suggests the answer is yes, and that
Earth and Mars cross-contaminated each other billions of years ago.
It's even conceivable that life began on Mars and travelled to Earth
some time later in meteorites, colonising our planet when conditions
eventually became favourable. If so, we are all descended from Martians!

The Garden of Eden - Revisited

Tantalising though these developments are, the problem remains
of how the first living cell formed. What chemical magic triggered
the vital spark? Whether the key process happened on Mars, Earth,
or both, the puzzle of the chemical genesis of life is still unsolved.
Armed with the new ideas, however, researchers are now focusing
their efforts on the chemistry of hot rocks infused with sea water.
Might the gases exuded by a planet's crust offer a more potent mix
than those of the primordial atmosphere? Could the pores in ocean
basalt play the role of tiny crucibles, in one of which was forged
that microbial Adam long, long ago? Some scientists think so, and
are increasingly hopeful that the key steps will soon be understood.

In many cultures the underworld has long been associated with
the realm of the dead. For Christians, it is the traditional location
of Hell, a place of fire and brimstone, and eternal torment. How
ironical if the torrid, sulphurous depths actually harboured the
cradle of life. Far from being Hell, the broiling bosom of our planet
might well turn out to have been the true Garden of Eden.

PAUL
DAVIES is an internationally acclaimed physicist, writer and broadcaster,
who holds the position of Professor of Natural Philosophy in the
Australian Centre for Astrobiology at Macquarie University, Sydney.
He has held previous academic appointments at the Universities of
Cambridge, London, Newcastle upon Tyne and Adelaide. His research
interests are in the fields of cosmology, quantum field theory,
and astrobiology. He is the author of over twenty books, including
The Mind of God, Other Worlds, God and the New Physics, The Edge
of Infinity, The Cosmic Blueprint, Are We Alone? The Fifth Miracle,
The Last Three Minutes, About Time, and How to Build a Time Machine.

Davies’s talent as a communicator
of science has been recognized in Australia by an Advance Australia
Award and two Eureka Prizes, and in the UK by the 2001 Kelvin Medal
and Prize by the Institute of Physics, and the 2002 Faraday Prize
by The Royal Society. For his contributions to the deeper implications
of science, Davies received the Templeton Prize in 1995.